Distillation Principles of Formaldehyde Solutions LIQUID-VAPOR EQUILIBRIUM AND EFFECT OF PARTIAL CONDENSATION Edgar L. Piret and M. W. Hall' University of Minnesota, Minneapolis, M i n n .
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aqueous solution is proportional only to the mole fraction of methylene glycol (formaldehyde monohydrate) in the solution (30). This property of formaldehyde also seems to be well established.
T h e distillation principles of aqueous formaldehyde solutions have been investigated. Included are data on the boiling points of aqueous solutions, the liquid-vapor equilibrium at atmospheric pressure as determined by two methods, the effect of fractionation of these solutions at atmospheric and at 20-mm. pressure, and the effect of partial condensation at atmospheric and at 500-mm. pressure. Experiments are described which clarify the discrepancies in the literature on the effect of distillation and on the formation of what have appeared to be constant boiling mixtures. I t is indicated that a favorable liquid-vapor equilibrium is not the cause of vapor enrichment.
This evidence that, in aqueous solution, formaldehyde exists as hydrates and that the partial pressure of formaldehyde is due only to the methylene glycol existing in solution is definite and in direct contrast to the uncertainty of the following experimental data relating to other properties of formaldehyde: Auerbach and Barschall ( 2 ) state that the boiling points of atmospheric pressure decrease constantly with increase in liquid concentration from 100" C. for a 0% solution to 99' C. for a 50% solution. Korzhev and Rossinskaya (14) indicate that the boiling points may pass through a minimum of about 98" C. for an 11 to 12y0 solution and thereafter increase. Walker (88) states that a boy0 solution boils a t 103' C. Auerbach and Barschall ( 2 ) have calculated the partial pressures a t the boiling point from their distillation data. The values obtained for the more concentrated solutions show large variations. Korzhev and Rossinskaya (14) have also reported partial pressure values calculated from distillation data for the limited concentration range of 0 to 26% formaldehyde. Their data are consistently higher than those of Auerbach and Barschall (a). Lacy (16) has developed an empirical partial pressure equation base! on the data of Ledbury and Blair (17) which were obtained a t 0 20°, 35O, and 45' C. and has extrapolated these data to 100' 'C. Walker (86) has noted that the Lacy equation gives values approximating the partial pressures for both aqueous formaldehyde solutions and paraformaldehyde for the tempera; ture range of 10" t o 60' C. but that it gives lower values a t 65 to 120" C. for paraformaidehyde than those obtained experimentally. There may be a question then whether the Lacy equation will hold for aqueous solutions a t the boiling point.
F
ORMALDEHYDE has become, in recent years, the key chemical in the manufacture of a large number of commercial products, including resins and plastics. The production of 40% formalin (8, 86) within the last two and a half decades has increased rapidly from 26,000,000 pounds in 1919, '52,000,000 pounds in 1933, and 181,000,000 pounds in 1940, to 522,000,000 pounds in 1944. This would seem to indicate that in the future. formaldehyde may become one of the world's heavy chemicals. The importance of formaldehyde as a modern commercial chemical warrants accurate and dependable information on its properties. The literature discloses, however, that the phenomena encountered in the distillation of aqueous formaldehyde solutions are very involved. While Walker (88) has done much to summarize the data in this field, a great deal of the available ififormation is still very contradictory and confusing. The extent of available knowledge of the properties involved in the distillation of aqueous formaldehyde solutions may be summarized as follows:
Liquid-vapor equilibrium data are very meager. Those of Auerbach a?d Barschall ( Z ) , reported in 1905, are apparently the only data available. It is uncertain from the description of their apparatus and procedure whether their data were obtained under equilibrium conditions. These data show large variations a t the higher concentrations. The state of formaldehyde in the vapor phase above its aqueous solutions has never been reported on the basis of direct experimental evidence.
As a result of an intensive study of such solutions, Auerbach and Barschall (8)showed that formaldehyde in aqueous solutions is hydrated, the dissolved formaldehyde being present as an equilibrium mixture of the monohydrate (methylene glycol) and a series of low molecular weight polymeric hydrates of the type (HCHO),HsO. The state of equilibrium is determined by the temperature and the formaldehyde content of the solution. High temperature and low formaldehyde concentration favor the methylene glycol; low temperature and high formaldehyde concentration favor the polymeric hydrates. Later work by Schou (22) on absorption spectra supports this study. Schou was 'able to estimate that the carbonyl group characteristic of unhydrated formaldehyde was present in aqueous solutions a t less than 1 in every 1200 formaldehyde molecules. Hibben (13) by means of Raman spectra confirms these data. That formaldehyde exists in aqueous solutions as methylene glycol (formaldehyde monohydrate) and higher hydrates seems t o be well established by confirmatory data of several investigators. A comparison of the vapor pressure data of Ledbury and Blair (17) for aqueous solutions with the Auerbach and Barschall data ( 2 ) for the methylene glycol content of aqueous solutions indicates definitely that the vapor pressure of formaldehyde in 1
b
Walker (SO) in 1931 shows by means of Clapeyron equation calculations that the vapor above aqueous solutions most probably is the unhydrated formaldehyde. Auerbach and Barschall ( 1 ) in 1907 performed some vapor density measurements on paraformaldehyde. The initial paraformaldehyde corresponding t o a composition of (HCHO)sH,O, when vaporized, gave a mean molecular weight of 29.3. Dissociation into seven monomeric formaldehyde and one methylene glycol molecules would give an apparent molecular weight of 32.2, whereas dissociation into eight monomeric formaldehyde and one water molecules would give an apparent molecular weight of 28.7, which would indicate that the paraformaldehyde molecules have dissociated for the most part into unhydrated monomeric formaldehyde and water. However, since Zimmerli (56 36) in 1927 expressed his opinion that methylene glycol (formaldehyde monohydrate) exists in the vapor phase and boils a t about 96" C., and that fractionation gives a vapor
Present address, Minnesota Mining and Mfg. Company, St. Paul,Minn.
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relatively rich in methylene glycol; and since Bond (6) in 1933 stated that vapors from aqueous solutions distilled a t atmospheric pressure contain little or no unhydrated formaldehyde, there seems to be some question as to the state of formaldehyde above aqueous solutions of 0 to 40ye concentration. Data given in the literature on the distillation of aqueous formaldehyde solutions a t atmospheric pressure are especially contradictory. This is shown by the following summary of the conclusions of ten investigators of the problem.
s
Auerbach and Barschall (2) in 1905 reported that when aqueous formaldehyde solutions were distilled at atmospheric pressure the distillate was always weaker than the residue. Wilkinson and Gibson (82) in 1921 presented data showing that when aqueous formaldehyde solutions are distilled at atmospheric pressure with a Hempel column as a still head, ah azeotrope exists containing about 8% formaldehyde. Above 87& the Figure 1. Boiling distillate is always weaker than the Point Apparatus residue, which fact is in qualitative agreement with Auerbach's ( 2 ) findings. Below S%, however, they show that the vapor is always richer than the residue. Blair and Taylor (4) in 1926 distilled a t atmospheric pressure aqueous formaldehyde solutions of 1 to 4570 concentration and indicated that an azeotrope exists containing about 30% formaldehyde. Zimmerli (35,36) in 1927 stated that i t is possible, by observing certain precautions, to distill weak solutions of formaldehyde of any concentration and always obtain a distillate of greater concentration than the residue. These data are in direct contradiction to the data of Auerbach and Barschall ( d ) , who found that distillates were always weaker than the residue. Furthermore, Zimmerli claimed that i t is possible to distill substantially all the formaldehyde, leaving behind nothing but water, obtaining solutions of any desired strength up to 50% concentration from solutions containing less than 1% formaldehyde. The method comprises refluxing a weak solution until equilibrium is reached in the column and then slowly distilling the formaldehyde. He states that in this way i t is possible to obtain a 90% yieldpf methylene glycol at the top of the column. On cooling, paraform is deposited immediately from the distillate. Ledbury and Blair (16) in 1927 reported that a 30% formaldehyde solution constituted an azeotrope. These investigators used a five-section Young and Thomas column for their distillations. Walker (31) in 1932 stated that by means of fractional condensation in an unpacked column with inserted bayonet-type condenser, aqueous formaldehyde solutions at atmospheric pressure could be distilled to give distillates that are always richer than the residue up to 53% formaldehyde, which is in agreement with what Zimmerli (36, 56) found. I n his examples, formaldehyde solutions are vaporized, using a reflux ratio of 12.5 to 1. Water vapor is condensed and removed as quickly as possible. I n this way, a %yoformaldehyde solution can be concentrated to give a 4070 formaldehyde solution containing 91.4y0 of the original formaldehyde in the charge, the first portions of the condensate being a 53.4% formaldehyde solution. Hasche (12) in 1932 distilled aqueous formaldehyde solutions at atmospheric pressure after adding a soluble substance such as calcium chloride to the dilute aqueous solution. The first runnings contained 61yo formaldehyde (methylene glycol is 62.5% formaldehyde). Bond (5)in 1933 stated that 30 to 33% formaldehyde solutions at atmospheric pressure form an azeotrope and that vapors from
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aqueous formaldehyde solutions distilled at low temperatures at atmospheric pressure contain little or no anhydrous formaldehyde. Korzhev and Rossinskaya (14)in 1935 stated that an 11to 12% solution of formaldehyde forms an azeotrope. At concentrations above 12%, the concentration of the formaldehyde in the vapor is less than the original solution, while a t concentrations below 8 to 9% the reverse holds true. These investigators also tried to concentrate aqueous formaldehyde solutions by adding calcium chloride to the solutions before distillation, but found that the distillate never exceeded 30 to 31% formaldehyde, whereas Hasche (12)obtained distjllates containing up to 61% formaldehyde. Reynolds (21) in 1937 stated that at atmospheric pressure aqueous formaldehyde solutions cannot be concentrated beyond about 23y0, because that per cent of formaldehyde acts as an azeotrope. He also stated that no such azeotrope exists when the solutions are distilled under reduced pressure. Examined individually, each of the distillation investigations seems to contain satisfactory data, and yet the discrepancies between the various data are very great. It is probable therefore that additional factors are involved. To date no one has clarified or explained the basis for these discrepancies. The information available in the literature and in the recent monograph on formaldehyde (28) does not explain why previous investigators could obtain various azeotropes of 8, 12, 23, 30, and 30 to 33y0 formaldehyde, why Zimmerli (35, 36) by fractionation and Walker (51) by fractional condensation could obtain distillates which are always stronger than the residue, or why -4uerbach and Barschall ( 2 ) could obtain distillates which were always weaker than the residue. Further study also seems warranted on the boiling points of aqueous solutions, the state of formaldehyde in the vapor, liquid-vapor equilibria, and eyuilibrium partial pressure data a t the boiling point. The present investigation was undertaken in an attempt to contribute to the clarification of these uncertain properties of formaldehyde solutions. MATERIALS USED AND METHODS OF AKALYSIS
Materials Used. The paraformaldehyde used was purchased from the D u Pont Company, which quotes the following specifications: minimum 957, formaldehytle by weight; the remaining 5% is principally combined water, not over 0.1% ash. The polymer melts in the range 120' to 150' C. in a sealed melting point tube. The authors' analysis of the sample showed 96% formaldehyde. The aqueous solutions were prepared by refluxing the 96y0 paraformaldehyde and distilled water until all the paraformaldehyde had dissolved. The solutions were then cooled, filtered, and aged for 7 days before analysis. The analysis of the aqueous solutions showed less than 0.02% free acid calculated as formic. Since Walker (28) and Wadano ( 2 7 ) have shown that even the
TABLE I. BOILINGPoIwrs OF AQUEOUS FORMALDEHYDE SOLUTIONS
W J . 70HCHO in Bolution
Distilled Water (control) Benzene (control) &!ethanol (oontrol) 2.57 4.46 7.98 12.62 15.5 19.8 22.65 26.35 31.1 41.5 43.8 45.8 47.0 47.2 49.3 51,. 5 53.1
Observed Boiling Point a t 740 Mm., 0
c.
99.3
...
9s:95 98.85 98.65 98.45 98.35 98.35 98.85 98.35 98.35 98.40 98.45 98.55 98.60 98.65 98.80 99.10 99.40
Observed Boiling Point a t 760 Mm.,
c.
100.0 80.05 64.70 99.75 99.60 99 . . .3 ..5 ~ 99.10 99.00 98.95 98.95 98.95 98.95 99.00 99.05 99.16 99.20 99.25 99.40 99,70 100.00
Standard Boiling Point a t 760 hlm.,
c.
100.0 80.08 64.65
.... ....
.... ....
.... .... .... ....
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It seemed desirable to extend this investigation beyond the commercial range of 0 to 37% formaldehyde and obtain boiling point data on aqueous solutions up to at least 50% formaldehyde. In this way it may be possible to clarify the two main discrepancies which seem to exist: Do the boiling points constantly decrease from 100" C. for a 0% solution to 99" C. for a 50% solution as stated by Auerbach and Barschall (W), or de the boiling points pass through a minimum a t 11 to 12% formaldehyde concentration and thereafter increase up to 103" C. for a 50% concentration as stated by Korzhev and Rossinskaya (14) and Walker (W8)? A satisfactory type of boiling point apparatus which is simple in design is that of Davis (Q), which is similar to the familiar Swietoslawski (24) type. The apparatus which was used is shown in Figure 1.
98.2
0
IO
20
30
40
50
60
WT% FORMALDEHYDE IN SOLUTION Figure 2. Boiling Points of Aqueous Formaldehyde Solutions New data at 760-mm. pressure data a t 740-mm. pressure X Auerbach and Barschall (2) corrected to 760 mm. A Korzhev and Rossinskaya (14) corrected t o 760 mm.
0 New
purest formaldehyde solutions are slightly acid and that, in general, the p H of pure aqueous formaldehyde solutions lies in the range of 2.5 to 3.5, these aqueous solutions may be considered as essentially acid-free. Neutral formaldehyde solutions can be obtained only by the addition of a buffer.
Methods of Analysis. Among the methods available for the determination of formaldehyde, the neutral sulfite method of Lemme (It?), improved by Doby ( I O ) and modified by Buchi (7), seems to be most satisfactory for the purpose. This method depends upon the condensation of formaldehyde with the sulfite ion and titration of the liberated hydroxide HCHO
+ SOa-- + H20
-
H C H ( 0 H ) SOI-
+ OH-
The analytical procedure used in this investigation is to dissolve a weighed pprtion of formaldehyde solution containing about 1 gram of formaldehyde in sufficient water to make 25 cc. of solution. T o this solution are added 3 drops of 1% thymolphthalein and the mixture is neutralized with 1 N sodium hydroxide until a blue colorappears. To this mixture is added a thymolphthalein-neutral 26y0 solution of sodium sulfite and the mixture is titrated t o complete discoloration with 1 N sulfuric acid. Each cubic centimeter of 1N sulfuric acid is equivalent ot 0.03003 gram of formaldehyde. I n com aring the numerous methods of formaldehyde analyses used on t i e same sample, Doby (10) finds the method of Lemme (18) most reliable. Borgstrom (6) shows that the sulfite method can be used in the presence of methanol, ethanol, formic acid, and methylal without error. Buchi ( 7 ) shows that in the presence of 27.28% ethanol, the maximum deviation in the sulfite analysis for formaldehyde is only +0.04%. Taufel and Wagner (26) show that 1%thymolphthalein affords a better end point than 1% phenolphthalein. I n all cases i t is necessary to make a blank run with sodium sulfite alone. The solution of sodium sulfite t o which thymolphthalein has been added must be neutralized immediately, because as shown thymolphthalein in contrast to phenolphthalein, by Seiler (W) is destroyed by 26% sodium sulkte solutions in a few minutes and then gives no blue color when sodium hydroxide is added. Carbon dioxide also counteracts the blue color. Therefore, the sulfite solution must be freshly prepared with carbon dioxide-free water. The solution should be neutralized a t once to colorless with 1 N sulfuric acid, then added to the neutralized formaldehyde solution for the final titration of the liberated sodium hydroxide.
The still, A , is a 500-cc. test tube to which a condenser, B, and tube C are added. C is the insert in which the Nichrome spiral heating element, D, is placed. D consists of 5 feet of No. 22 Nichrome wire wound on a piece of 10-mm. glass tubing. The Cottrell pum E, is an inverted funnel of 30-mm. glass tubing which is liftecklightly off t6e bottom of still A . When D is turned on, the heat is applied direct1 beneath the funnel, so that the boiling liquid is forced upwardiand impinges on the bulb of the thermometer, F . The thermometer was graduated in 0.1 O C. and temperatures were estimated to 0.05' C. E is made large enough so that all boiling occurs under the funnel. I n operation, the apparatus is filled with 110 cc. of solution to about the level of the return spout below F. The Nichrome heating element, D, is then turned on. That there is practically no superheating was shown by the fact that doubling the heat input caused no detectable change in the observed temperature. As suggested by Swietoslawski (24) the thermometer, F, and the apparatus were checked by observing the boiling points of three pure liquids: distilled water, benzene, and methanol. The observed temperatures checked the recognized values for the boiling points within 0.05' C. Data were obtained at controlled pressures of 760 and 740 mm. of mercury for a series of concentrations ranging from 0 to 53y0 formaldehyde. The results, as shown in Table I and Figure 2, indicate that the boiling points of aqueous formaldehyde solutions, at 760-mm. pressure, decrease with increasing formaldehyde concentration from 100.0' C. for 0% solutions to a minimum of 98.95' C. for the concentration range of 20 to 35% formaldehyde and then increase to 100.0' C. for a 53.1% f o r m a l d e hyde solution. The boiling point data of Auerbach and (2) Barschall and of Korzhev and Rossinskaya (14) were corrected to 760mm. pressure and plotted in Figure 2. The d a t a of t h e former are generally higher than the new data. The error in the former author's conclusion that the boiling points constantly deFigure 3. Differential Liquid Vapor crease with in. Equilibrium Apparatus creasing concen-
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M T A AT 760 MM. PRESSURE
0-NEW DATA AT
740MM. PRESSURE
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known methods require an appreciable temperature gradient, with change of composition, they do not seem to be so suitable for equilibrium measurements on mixtures such as aqueous formaldehyde solutions, the boiling point,s of which vary only slightly with change of composition. Because the apparatus and the conditions used in the distillation of formaldehyde solutions have been shown to have such pronounced effects on the results obtained, the apparatus and procedures used here are described in some detail. Differential Equilibrium Method of Zawidski (34). The differential equilibrium apparatus used in this study !s a modification of that of Zawidski (34) and is shown in Figure 3.
X-DATA OF UlERBACH LI BARSCHALL
The still, A , 4s a 1-liter 3-necked roundbottomed flask with ground-glass connections. Tube B is the vapor outlet which extends 6 inches above the surface of the oil bath and wT.9'0 FORMALDEHYDE I N LIQUID then leads downward to condenser E . The Figure 4. Liquid Vapor Equilibria of .4queous Formaldehyde Solutions upright portion of B is wound with a Kichrome wire heater element,, C, consisting of 5 feet of KO.22 Yichrome wire. n-hich extends slichtlv below the surface of t,he oil bath. B extknds to wit,hin 3 igches trat.ion would. have become apparent if addit,ional data had of the equilibrium mixture. C is heated electrically to 110" to been obtained for the concentration range of 42 to 53%, form130 C. to evaporate any reflux condensate which forms in vapor aldehyde. The data of Korzhev and Rossinslraya (14) for tube B . At no time during any of the runs did a single drop of rethe limited concentration range of 0 to 2670 formaldehyde flux run back into the liquid in A . The vapor is therefore superheated, but only after it,has entered B sufficiently far removed from follox the same trend as the new data, but arc about 0.2" to the liquid in A so that it would not' affect the equilibrium condi0.3" C. lower. Since they apparently used a thermometer which tions. Thermometer D records the temperature of t,heliquidin the could be read only t,o 0.5" C., it is reasonable t,o'assume t,hat their still which is heated internally by an electric heating element, G. data arc not in disagreement. The boiling point of a 50Y0 The internal heater, G, consists of 5 feet of Yo. 22 Nichrome wire wound on a piece of glass tubing, J . The lead-in wires are of solution is shown to be 99.5" C., which is appreciably lower than copper. J fits into the outer tube, H , which extends nearlv to the value of 103 C. given by Walker ($8). the bott.om of A . The heating element is arranged so t,hat i t is The variation in boiling point. of these solut,ions i s probably due completely below the surface of the formaldehyde solution at all to the proportions of water, methylene glycol, and the higher times. The annular space abovc G and between tubes H and J is filled with asbestos, F , so t,hat no signjficant amount of heat hydrates which exist a t each concent>ration. At low concenis transmitted above t,he level of the liquid t o superheat the vatrations the solutions consist principally of methylene glycol pors. and water. a s the methylene glycol content of the solutions In a preliminary trial H above the surface of the liquid was increases, t.he boiling points decrease. Counterbalancing this found to be scarcely %-armto the hand even when the heating decrease in boiling point, however, is the increasing cont,ent of element was at a red heat. During the distillat'ion experiments the heater wircs wcrc at the lower temperat,ure and never showed the nonvolatile higher hydrates. These predominate in the color. For all the runs 20 volts were used on the internal heater; more concentrated solut,ions and account for the eventual rise in this was sufficient to distill the mixtures at the rate of drop boiling point. per second. The thermometer, D , the electric heating element, G, and the vapor outlet tube, B, are inserted in the 3 ground-glass connections on A and sealed with a solution of polyvinyl acet,ate LIQUID VAPOR EQUILIBRIA O F AQUEOUS FORMALDEHYDE to prevent the light oil from penet'rating the glass connections. SOLUTIONS A is totally immersed in a bath of light petroleum oil which is heated elect,rically. The temperahre of t,he oil bath is kept 1 O to The separation by rectification of the constituents of a binary 3" C. above t,he temperature of the liquid in the st,ill which is solution and the design of suitable distillation apparatus for such recorded by D. This difference in temperature is so small that separation are ordinarily dependent on the relationship between t,here could hardly have been any superheating of the contents of the composition of the solution and the composition of the vapors the still. The bath is agitated by an electric stirrer. which are in equilibrium with the solution. It was felt that I n order to permit the mixtures to attain equilibrium before although nonequilibrium conditions might be particularly imdistillation, the bath was maintained at 99" to 100" C. for an portant in the formaldehyde system, a study under equilibrium initial period of 1 hour in the case of the solutions containing conditions was essential as a basis for further work There are a number of different methods available for the 0 to 22% formaldehyde and 2 hours in the case of solutions containing 22 to 50y0 formaldehyde. The internal heater, G, and determination of liquid-vapor equilibrium data of binary liquid the vapor heater, C, were then turned on and the mixtures dismixtures. Of these, the differential distillation method of tilled. Approximately every 15 minutes the fraction distilled, Zawidski (34) and the recirculation method of Othmer (19) weighing about 20 grams, was removed and analyzed. From were chosen for this investigation. The two methods were used four to eight separate fractions were obtained from the distillation in order to ensure the validity of the results obtained. Othmer of each sample'. Consecutive samples were taken without an (19, 20), Young, (SS), and Griswold, Andres, and Klien (11) intermediate equilibrium period between samples. The first have listed the more important methods and outlined the redistillate in all runs was analyzedfor material balance calculations. quirements for a satisfactory equilibrium still. They have Since a few drops of the solution had vaporized and distilled discussed the inaccuracies inherent in these methods and have over before the vapor heater was turned on, this first distillate pointed out that if proper precautions are taken, both methods was not used as equilibrium data. are capable of very good results. Since the remaining better A-DATA
O
OF
KORZHEV L ROSSINSKAYA
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Each distillate above 12y0 was analyzed in duplicate. If differing by more than 0.05%, it was checked a third time. .Below 12% the whole distillate fraction was used for the analysis,
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but the accuracy was checked by the analysis of the succeeding distillate fractions. The residue was analyzed initially and at the end of each run. The solution charged to the still was I accurately weighed and the analysis of the intermediate residues determined by mateTABLE 11. DISTRIBUTION OF FORMALDEHYDE BETWBEN LIQUIDAND VAPORBY BOILING rial balance calculations. In A4QUEOUSFORMALDEHYDE SOLUTIONS every case the calculated and Total the analyzed value of the resiEquil. Bath Liquid Wt. Wt. % in Liquid Period, Barom., Temp., Temp., Grams of Solution % in due agreed within 0.1%. Hours Mm. Hg OC. Before After Vapor Before OC. After Av. Vapor The data are given in Table I1 1.25 733 101 918.6 892 0 4.46 99.7 26.6 4.46 4.46 4.16 1.50 733 102 892.0 4.46 875.0 99.7 17.0 4.46 4.46 4.14 and shown in Figure 4. All 1.75 733 875.0 4.46 852.0 101 99.7 23.0 4.47 4.46 4.12 -the distillations using this 4.46 4.14 equilibrium still were carried 3.25 740.1 736 100.5 99.7 720.0 20.1 7.90 7.91 7.90 7.33 out at prevailing atmospheric 3.50 720.0 736 100.5 99.7 699.4 20.6 7.91 7.93 7.92 7.28 3.75 699.4 736 100.5 99.7 676.4 23.0 7.93 7.96 7.95 7.16 pressure, which was usually 4.00 676.4 736 100.5 99.7 657.8 18.6 7.96 7.97 7.96 7.27 close to 740 mm. The lower 7.93 7.30 line shown in Figure 4 was 1.22 740 100 99.8 948.0 935.0 13.0 7.99 7.99 7.99 7.35 drawn to represent the equilib1.45 740 100 919.0 99.8 935.0 16.0 7.99 8.00 7.99 7.28 1.67 740 919.0 100 99.8 14.4 8.00 904.6 8.01 8.00 7.29 rium curve for an average pres1.93 740 99.8 904.6 100 18.3 886.3 8.02 8.01 7.27 8.01 __ sure of 740 mm. 8.00 7.30 That the time of heating before 1.25 741 100 99.7 979.0 957.7 21.3 12.04 12.06 12.05 10.6 distillation was sufficiently long 1.50 741 100 99.7 957.7 936.9 20.8 12.06 12.10 12.08 10.6 1.75 74 1 100 99.7 936.9 914.8 22.1 12.10 12.15 12.12 10.65 to ensure equilibrium is shown 2.00 99.7 914.8 74 1 100 19.8 12.15 895.0 12.16 12.18 10.6 by the several check runs in 12.1 10.6 which the solutions were held 1.42 745 100 99.8 974.0 944.0 30.0 15.20 15.25 15.23 13.2 a t 99" to 100" C. for longer 745 1.67 101 99.8 944.0 922.5 21.5 15.25 15.26 15.25 13.1 745 2.08 922.5 100.5 99.8 892.5 15.38 15.32 30.0 15.26 13.3 periods of time. For instance, -the approximate 8% solutions 15.3 13.2 were checked by holding these 27.25 ,740 101 99.7 647.6 625.2 22.4 19 ..3 19.4 19.35 16.4 27.50 740 101 625.2 99.7 604.4 20.8 19.4 19.5 19.45 16.45 a t 99" to 100" C. for 1 and 3 27.75 740 101 604.4 99.7 583.2 21.2 19.5 19.6 19.55 16.4 28.00 740 101 99.7 583.2 562.6 20.6 19.6 19.7 19,65 16.4 hours, respectively, before distillation, while the approximate 19.5 16.4 2.25 742 99.5 99.4 901.7 882.1 19.6 19.6 19.7 19.65 16.35 20% sample was held a t 99" 2.42 742 99.5 99.4 882.1 869.1 12.8 19.7 19.75 19.7 16.55 to 100" C. for 1, 2, and 27 2.58 742 99.9 869.3 103 855.3 14.0 19.75 19.8 19.8 16.65 2.75 742 104 855.3 100.0 832.8 22 5 19.8 19.85 19.85 16.7 hours, respectively, before distillation. The points obtained 19.75 16.6 1.16 737 99.5 99.4 965.6 948.0 17.6 19.9 19.95 19.9 in each case for the longer 17.1 1.42 737 100 99.6 948.0 932.5 15.5 19.95 20.0 20.0 17.05 heating periods were very close 1.67 737 100 99.6 932.5 912.0 20.5 20.0 20.05 20.0 16.95 1.92 737 100 912.0 99.6 890.6 21.4 20.05 20.15 20.1 16.85 to the runs in which the sam2.16 737 100 99.6 890.6 20.4 20.15 870.2 20.2 20.2 16.85 2.27 737 100 99.6 870.2 20.2 862.0 8.2 20.3 20.25 16.9 ples were held for only 1 and 2 hours. 20.1 -16.95 That equilibrium was fully 2.25 73 1 100 99.4 971.3 949.0 22.3 25.6 25.7 25.65 21.3 2.50 731 100 99.4 949.0 926.0 23 0 25.7 25.8 25.75 21.3 maintained throughout the 2.75 731 100 99.4 926.0 906.0 20 0 25.8 25.9 25.85 21.45 3.00 731 906,O 100 99.4 886.3 19.7 25.9 26.0 25.95 21.65 course of each distillation is 3.25 731 100.5 99.4 865.0 886.3 21.3 26.0 26.1 26.05 21.55 shown also by Table I1 and -25.85 21.45 Figure 4. Below 16y0 the 2.25 738 99.5 99.4 982.1 961 6 20.5 29.6 29.7 29.65 23.65 several points obtained during 2.50 738 100 99.5 961.6 939.1 22.5 29.7 29.85 29.75 23.75 2.75 738 100 99.5 939.1 916.8 22.3 29.85 20.0 29.9 23.8 each run duplicate themselves 3.00 738 99.3 99.5 916.8 897.8 19.0 30.0 30.1 30.05 23.95 3.25 738 99.3 99.5 897.8 883.0 very well. Above 16% the 30.1 14.8 30.25 30.2 24.0 -several points obtained are ,29.9 23.85 more spread out, but are still 2.25 750 100 99.7 949.7 935.7 14.0 31.65 31.7 31.8 25 3 2.50 750 100 99.7 935.7 916.7 31.8 19.0 32.0 31.9 25.3 close t o the equilibrium curve. 2.75 750 100 99.7 916.7 899.4 17.3 32.0 32.05 32.05 25.3 750 3.50 100 99.7 899.4 851.8 47.6 32.05 32.35 32.2 25.35 This is due t o the fact that -with increasing concentration of 31.95 25.3 2.30 745 100 99.5 983.3 964.7 18.6 35.2 35.4 the initial solution the difference 35.3 27 3 2.58 745 100 99.5 964.7 945.5 19.2 35 4 35.5 35.45. 27.2 between the vapor and the 2.83 745 100 99.5 945.5 931.1 14.4 35.5 35.7 35.6 27.45 3.08 745 100 931.1 * 99.5 912.3 35.7 18.8 35.85 35.8 27.5 liquid compositions becomes 3.75 745 100 99.5 912.3 863.4 48.9 35.85 36.3 36.1 27.5 greater and the intermediate 35.65 27.4 residues show a more rapid in2.25 750 100 99.6 978.0 959 8 18.2 38.4 38.6 38.5 e9.0 2.50 750 100 crease in concentration. 99.6 959.8 939 .O 20.8 38.6 38.8 38.7 29.0 2.75 750 100 99.6 939.0 922.5 16.5 38.8 38.95 38.9 29.2 Recirculation Equilibrium 3.00 750 100 99.6 922.5 902.5 20.0 38.95 39.15 39.05 29.15 3.25 750 100 99.6 902.5 889.0 13.5 39.15 39.33 39.25 28.9 Method of Othmer (19). The Othmer recirculation type of 38.8 29.1 4.25 736 104.5 103.0 886.0 870.5 12.9 49.15 49.45 equilibrium apparatus used in 49.3 33.7 4.50 736 104.5 103.1 870.5 855.5 15.6 49.45 49.8 49.6 33.9 4.75 this study is a modification of 736 104.5 103.1 855.5 839.0 49.8 14.9 50.1 49.95 34 2 5.00 736 104.5 103.1 839.0 824.0 16.25 50.1 50.4 50.25 34.3 that of Baker, Hubbard, Huguet, and Michalowski (3) and is 49.8 34.0 shown in Figure 5.
Gy&fms
I
--
.
INDUSTRIAL AND ENGINEERING CHEMISTRY
666
Vol. 40, No. 4
TABLE 111. DISTRIBUTION OF FORMALDEHYDE BETWEEN LIQUIDAND VAPORBY BOILING AQUEOUSFORMALDEHYDE SOLUTIOKS Total Equil Period, Hours 40.0 40.25 40.5
Press. in System, Mm. Hg 740 740 740
Bath Temp., 102 103.5 102
99.9 99.9 99.9
109 109 109
Grams of Solution Before After 873.3 847.1 847.1 819.9 819.9 793.7
40.75 41 .O 41.25 41.5 41.75
760 760 760 760 760
101 5 102 5 102.5 102.5 102
100.6 100.6 100.6
100.6 100.6
109 109 109 108 109
793.7 769.7 745.5 718.6 692.8
769.7 745.5 718.6 692.8 670.4
24.03 24.14 26.91 25.82 22.37
3.89 3.89 3.89 3.89 3.90
3.89 3.89 3.85 3.90 3.90
3.88 3.89 3.89 3.89 3.89 3.90
3.66 3.86 3.87 3.83 3.81 3.84
42.0 42.33 42.58 42.83
740 740 740 740
105 102.5 102.5 102.5
100 99.9 9s.9 99.9
110 109 109 109
670.4 632.3 606.1 580.4
632.3 606.1 580.4 553.6
38.14 26.13 25.71 26.81
3.90 3.92 3.94 3.95
3.92 3.94 3.95 3.96
3.89 3.91 3.93 3.95 3.95
3.84 3.68 3.65 3.71 3.69 3.68 7.37 7.40 7.41
OC.
Liquid Temp., OC.
Vapoi Temp. OC.
Grams of Vapor 26.21 27.18 26.25
Wt. Toin Liquid Before After Av. 3.88 3.88 3.88 3.88 3.88 3.88 3.88 3.89 3.88
Wt. yo in Vapor 3.65 3.65 3.68
---
99.5 99.9 99.9
110 108 108
879.0 866.4 853.5
866.4 853.9 841.1
12.62 12.52 12.79
7.94 7.95 7.96
7.95 7.96 7.97
3.95 7.95 7.95 7.96
102 102 102
100.6 100.5 100.5
105 109 109
841.1 827.0 814.6
827.0 814.6 803.3
14.05 12.37 11.25
7.97 7.98 7.98
7.98 7.58 7.99
7.95 7.97 7.98 7.98
7.40 7.67 7.71 7.70
760 760 760
101.5 103.0 103
100.2 100.2 100.2
108 108 108
795.0 790.0 781.8
790.0 781.8 772.6
9.0
8.2 9.2
22.4 22.44 22 48
22.44 22.48 22.51
7.98 22.42 22.45 22. 50
7.69 19.2 19.2 19.2
2.67 3.23 3.78
744 744 744
103 102.5 102.5
99.8 99.8 99.8
105 107 107
772.6 765.5 759.0
765.5 759 .O 751.9
7.1 6.5 7.1
22,Zl 22,55 22.58
22,65 22.58 22.60
22.45 22.53 22.56 22.55
19.2 18.95 18.95 19.05
0.5 0.83 1.16 1.75
760 760 760 760
103.5 103.5 103.5 103
100.6 100.6 100.6 100.6
108 109 108.5 105
919.0 910.3 901.8 892.7
510.3 901.8 892.7 883,5
8.7 8.5 9.1 9.2
30,65
30.66 30.7 30.8 30.9
22.55 30.65 30.68 30.75 30.85
19.0 24.85 24.9 24.95 24.95
0.25 0.92 1.34
743 743 743
101.5 101.5 101 5
1.75 1.92 2.08
760 760 760
1.0 1.55 2.1
30.66 30.7 30.8
-_
--
24.9 24.7 24.7 24.75 24.85
2.08 2.53 2.88 3.25
737 737 737 737
102.5 103 103 103
99.8 99.9 100.0 100.0
105 109 109 109
883.5 861.5 850.7 839.0
861.5 850.7 839.0 824.9
22.0 10.8 11.4 14.1
31.0 31.0 31.1 31.2
31.0 31.1 31.2 31.3
30.75 30.95 31.05 31.15 31.26
0.5 1.08 1.66
760 760 760
104 104 104,5
101.7 101.7 101.7
108 108 108
914.0 906.7 899.1
906.7 899.1 891 .o
7.3 7.6 8.1
41.5 41.55 41.66
41.55 41.65 41.8
31. 1 41.5 41.6 41.7
24.75 30.55 30.9 30.75
2.26 2.84 3.44
733 733 733
104 103.5 104.
100.9 100.9 100.9
108 108 108
891.0 883.4 874.2
883.4 874.2 863.8
7.6 9.2 10.4
41.8 41.9 42.1
41.9 42.1 42.2
41.6 41.85 42.0 42.15
30.75 30.45 30.5 30.5
1.25 1.28 1.56 1.59 1.62 1.67
760 760 760 760 760 760
105 106.5 104 104.5 104.5 104.5
103 103.2 103.1 103.1 103.1 103.1
111
112 112 112 112
915.1 912.4 909.1 905.6 903.0 899.2
912.4 509.1 905.6 903.0 899.2 895.0
2.71 3.31 3.52 2.52 3.81 4.15
47.2 47.2 47.25 47.25 47.25 47.3
47.2 47.25 47.25 47.25 47.3 47.4
42.0 47.2 47.22 47.25 47.25 47.28 47.35
30.5 33.05 33.25 33.26 33.23 33.45 33.45
2.22 2.28
743 743
102.5 103.5
102.1 102.4
105 107
855.0 891.4
891.4 887.7
3.62 3.73
47.4 47.5
47.5 47.55
47.25 47.45 47.52
33.3 33.05 33.1
47.5
33.1
111
The still, A , is a 1-liter 2-necked round-bottomed flask with ground-glass connections. To this flask are fused a 15-mm. vapor outlet tube, B , and a 10-mm. condensate return tube, K . B is the column characteristic of the Othmer apparatus and contains the specially constructed still head, C, to remove as distillate any vapors condensed in the upper portion. This still head prevents the return of reflux to the surface of the still liquid, thereby eliminating any errors due to partial condensation in C. Thermometer M records the temperature of the vapors. Condenser E condenses the vapors and delivers them to a small receiver, P. Any condensate formed in C also goes through E to receiver P. Except when vapor samples are being withdrawn for analysis, the condensate in P goes through the t,hree-way cock, N , and is returned t o the still a t K . No condenser is necessary between K and A , since the boiling point of the condensate is essentially the same a i the boiling point of the liquid in the still. Thermometer D records the temperature of the liquid in the still, which is heated internally by an electric heating element, G. Both D and G are
,
inserted in the two ground-glass connections on A and sealed with a solution of polyvinyl acetate to prevent the light oil from penetrating the glass connections. The internal heater, G, is the same as that used for the still shown n Figure 3. Fifteen to 20 volts on G are used on all the runs. !&is voltage is sufficient to distill the mixtures a t a rate of '/3 to 1/2 drop per second. A is totally immersed in a bath of light petroleum oil which is heated electrically. The temperature of the oil bath is kept at 1O to 3 O C. above the temperature on D. The bath is agitated with an electric stirrer. The vapor outlet tube, B, extends 24 inches above the surface of the oil bath. B is wound with a Nichrome wire heating element, Q , consisting of 12 feet of No. 24 Nichrome wire which extends slightly below the surface of the oil bath and to the top of the still head, C. The vapor temperature is maintained a t 108' t o 112 ' C. at all times. Since no refluxing takes place, the distillate collected in P should be in equilibrium with the large volume of liquid in A . Container U is a 3-gallon carboy which is used to minimize
INDUSTRIAL AND ENGINEERING. CHEMISTRY
April 1948
4
Figure 5. Recirculation (0thmer) Liquid Vapor Equilibrium Apparatus
667
for specified periods of time seems t o be just as satisfactory as maintaining the samples a t the boiling point as used in the recirculation still. In both stills the measured liquid temperatures are apparently affected by varying amounts of superheat so as to be higher than those of Figure 2. The liquid-vapor equilibrium data obtained indicate that when aqueous formaldehyde solutions are distilled at pressures of 731 to 760 mm. under equilibrium conditions where reflux condensation is prevented, the distillate is always weaker than the residue. This confirms the conclusion of Auerbach and Barschall
(2). Partial Pressure of Aqueous Formaldehyde Solutions at Boiling Point. The partial pressures of aqueous formaldehyde solutions a t the boiling point, for pressures varying from 731 to 760 mm., have been calculated from the liquid-vapor equilib-
pressure variations in the still. There are four leads to the carboy. Through lead R A small volume of compressed air is introduced to maintain the pressure. Lead 8 connects to barometer W , which indicates the pressure in the system. Lead 2 connects to a piece of 7-mm. glass tubing which can be adjusted to any depth in the glass container, V . The air from the carboy flows through 2. By raising or lowering this outlet, the pressure in the system can be lowered or raised. The other lead, T, connects to bulb P, so that the distillation pressure can be maintained a t 760 mm. with a variation in pressure of less than 1 mm. A procedure was used which was similar to but not identical with that used in the differential method. For each concentration of formaldehyde the distillation was carried out a t 760-mm. pressure and also a t prevailing atmospheric pressure. The still was run for the specified equilibrium period before samples were withdrawn. Each mixture was distilled slowly a t a rate of about 1/3 drop per second. Every 2 to 15 minutes, the fraction distilled weighing 3 to 20 grams was removed and analyzed. From three to five sepltrate fractions were obtained a t each pressure from the distillation of each sample. Each distillate above 12% was weighed and then analyzed in duplicate, Below 12% the whole distillate fraction was used for the analysis, but the accuracy is checked by the analyses of the succeeding distillate fractions. The residue was analyzed initially and a t the end of each run. The solution charged to the still was accurately weighed before and after the run and the analyses of the intermediate residues were determined by material balance calculations. In every case the calculated and the analyzed formaldehyde contents of the residue agreed within 0.1%. The initial equilibrium period was varied from 15 minutes t o 40 hours and the intermediate equilibrium period allowed between samples was varied from 0 to 30 minutes. During the equilibrium periods the solutions were maintained a t the boiling point by distillation with subsequent condensation and return of distillate to the still. The results obtained with the Othmer recirculation type of equilibrium still, which are shown in Table 111 and Figure 4, duplicate very well those obtained with the differential type of still. That equilibrium was attained initially and also fully maintained throughout each of the runs is indicated by the fact that the same results were obtained in the initial sample fractions as in succeeding sample fractions, even though various intermediate equilibrium periods were used. It is apparent also that the equilibrium method used for the differential still of maintaining the samples a t 99" to 100' C.
rium data and are shown in Table IV. These computations are on the basis that the formaldehyde was uncombined and that Dalton's law held. The new partial pressure data and also those of Auerbach and Barschall @) and Korahev and Rossinskaya ( I d ) were corrected to 760-mm. pressure in order t o correlate these data with the Lacy equation. These corrected data are plotted in Figure 6. The new data and the Lacy calculations correlate nicely for the range of 0 to 20% concentration, then diverge, the Lacy calculations giving lower values at concentrations of 20 to 40% formaldehyde. The L ~ c yequation was also found by Walker (28)to give low values for paraformaldehyde a t the higher temperatures. The new partial pressure data are in fairly good agreement with the values of Auerbach and Barschall (9) for low formaldehyde concentrations, but are much higher than their values a t the higher concentrations. The new data are consistently lower than the partial pressure data of Korzhev and Rossinskaya (14). It is apparent, however, from the comments of Korzhev and Rossinskaya regarding the formation of droplets 8
0-NEW
I
DATA OBTAINED AT 780 MM.
0-NEW DATA CORRECTED TO 760 M M.
1",
ov
0
Figure 6.
II
II
X-DATA OF AUERWH 6 BMISCHALL CORRECTED TO 7 0 MM. A-DATA oc KORZHN a ROSSINSKAYA CORRECTED TO 7W MM. 0-LACY
.
EQUATION
IO 20 30 40 50 W.Yo FORMALDEHYDE I N LlQUl D
II
80
Partial Pressurea of Formaldehyde at Boiling Point
668
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 40, No. 4
formaldehyde content of the liquid condensate. Under such TABLE IV. PARTIAL PRESSURE OF AQUEOUS FORMALDBHYDE conditions Figure 4 would not apply. SOLUTIONS AT THE BOILING POINT Since so many investigators have shown that, on a total Mole Ca1cd.a Partial formaldehyde basis, dist'illates richer than the residue can be Fraction Partial Pressure Wt. % ' HCHo HCHO in Pressure, Barom., Cor. to obtained, it appears probable that fractional distillation is In liquid I n vapor Vapor Mm. H g Mm. Hg 760 Mm. Hg effective in some way for the enrichment of aqueous formaldehyde 3.66 0.0223 16.5 740 17.5 3.88 vapors. Fract,ional distillation alone, however, does not explain 3.89 3.84 0.0234 17.8 760 17.8 34 .. 94 56 00 .. 00 22 2543 1 740 why various azeot.ropes exist or why some investigators obtained 43 .. 16 48 1 68 .. 65 733 21 07 .. 55 7.93 7.30 0.0452 33.2 736 35.5 no azeotropes but found instead that the distillates were always 7.95 7.40 0.0458 34.0 743 35.5 richer than the residue. These contradictions indicate t,hat 7.98 7.69 0.0476 56.2 760 36.2 740 8.00 7.30 0.0452 33.5 35.5 some other factor may be involved. 0.0665 49.3 741 52.5 12.1 10.6 15.3 13.2 0.0837 62.3 745 65.0 I t seemed desirable to demonstrate once again that it is possible 19.5 16.4 0,1055 78.0 740 81.5 0.107 79.4 742 82.5 under conditions wherein fractionation is employed, for the 19.75 16.6 22 02 .. 41 5 00 .. 11 20 59 737 vapors to be richer than the residue; then, to investigate the 11 96 .. 92 5 08 05 .. 30 760 98 54 .. 00 22.55 19.0 0.1235 92.0 744 95.0 variation in vapor composition with various degrees of fractiona25.85 21.45 0.141 103.0 731 108.5 0.158 116.5 738 120.5 tion or fractional condensation; and finally to demonstrate the 29.9 23.85 00 .. 11 66 65 760 reasons for the contradictions between the various investigators. 737 11 22 76 .. 00 33 01 .. 715 22 44 .. 79 5 11 22 26 .. 00 31.95 25.3 0.1685 126.5 750 129.0 The distillation method of Wilkinson and Gibson (32) was 745 141.0 35.65 27.4 0.1845 137.5 0,1975 148.0 750 160.5 chosen, wherein some degree of fractionation could occur and 38.8 29.1 44 12 .. 06 33 00 ..755 760 enrichment again be demonstrated. 00 ., 22 1018 11 56 20 .. 50 733 11 66 00 .. 05 47.25 47.5 49.8 Assuming
Q
33.3 0.230 175.0 760 33.1 0,229 170.0 743 34.0 0.236 174.0 736 Dalton's lam and that vapor is dissociated.
175.0 175.5 181 .o
in the upper portion of their apparatus,,that condensation of the vapors occurred during their distillations. As will be demonstrated later, such conditions result in partial pressure values which are too high. DEMONSTRATION OF FRACTIONAL DISTILLATION PHENOMENA CAUSING APPARENT DISCREPANCIES
I n this section will be shown the phenomena causing the discrepancies among the results and conclusions of previous investiffators in their studies of the distillaOf aqueous dehyde solutions. The new liquid-vapor equilibrium data in Figure 4 show that simple equilibrium distillation does give that are always weaker than the residue. At. first thought it would seem probable t,hat fractionation of t,hevapors would cause the distillates to become weaker and weaker in formaldehyde. It must be remembered, however, that the liquidvapor equilibrium curve is based on tot'al formaldehyde and not on methylene glycol. Figure 7. Fractional The amount of forDistillation Apparatus maldehyde present in the form of methylene glycol in any aqueous solution a t equilibrium is less than the total formaldehyde cont'ent' beeahse of the presence of varying amounts of the higher hydrates. It might be possible, however, during the fractionation of the vapors, under nonequilibrium conditions when time is not allowed for the formation of appreciable polymer in the liquid condensate, for vapor enrichment to occur. The methylene glycol content of the vapor would then be great,er than the methylene glycol content or even the total
The apparatus shown in Figure 7 consists of a 1-liter roundbottomed single-necked flask, A , for a still, a 225-mm. Hempel column, .B, packed with 6-mm. glass Raschig rings, and a Liebig condenser, C. The column was uninsulated to allow reflux to form. Each mixture was boiled for 30 t o 40 minutes to attain equilibrium, condenser C being tipped so t.hat all the condensate ran back through the Hempel column, B, into the still, A. When distillation was begun, the distillate fractions were analyzed as before. The data are shown in Table V and Figure 8. The relatively smooth curve obtained s h o m that an azeotrope apparently exists containing 12y0 formaldehyde. This is similar to t h e . results obtained by Wilkinson and Gibson (32). The curve is similar in shape to the liquid-vapor equilibrium curve, but is raised slightly so that it the 450 diagonal at 12% formaldehyde. These data the previous data, and show that fractionation is effective as a means of increasing the formaldehyde content of the vapors. The apparatus shown in Figure 9 was then uscd to demonstrate the variation in vapor composit.ion which occurs when various degrees of partial condensation are employed. '
The st,ill, A , consists of a 1-liter round-bottomed single-necked flask to which a short length of 10-mm. glass t.ubing, B , is fused. The vapor tube, C, which connects still A and colunin D is bent down from the horizontal, so that the reflux condensate which
5
'x r:
3
I
Figure 8.
WT.% FORMAUXHYDE IN LIQUID Fractional Distillation of Aqueous Formaldehyde Solutions
April 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
forms is diverted from direct return to A . The 40-inch wetted wall tower, D , of glass tubing 25 mm. in inside diameter is exposed to the air and therefore air-cooled so that the rate of partial condensation remains practically constant. The hot reflux condensate which runs down the walls of D passes through the U-tube, E , and the three-way stopcock, F , and finally returns to the still through B. Whenever a sample of the reflux condensate is desired, it is obtained through the three-way stopcock, F , tube G , and condenser H . The uncondensed vapors (the distillate) pass out column D through tube J and are totally condensed in condenser K. The distillate passes through the three-way stopcock L, tube M , and finally returns to the still through B. Whenever a sample of distillate is desired, i t is obtained through L and tube N . The temperatures of the liquid and vapor are measured by thermometers P and R , respectively. A is heated electrically by a spherical Glas-Col mantle, &, which minimizes and almost eliminates any partial condensation in A . Since the cooling area of D and the temperature difference due t o air cooling remain constant, the weight rate of reflux condensate remains relatively constant. The rate of ' distillation and therefore the reflux ratio are changed by varying the heat input t o the GlasCol heating mantle, Q. The procedure followed was t o distill a t
TABLE V Barom., Mm. H g 740 740 740 740
DISTILLATION OF AQUEOUSFORMALDEHYDE SOLUTIONS AT ATMOSPHERIC PRESSURZ WITH RECTIFICATION Distillation Time Min.' 15 15 15 15
Grams of Solution Before After 366.7 342.4 342.4 320.8 320.8 287.0 287.0 254.3
Grams of Vapor 24.3 21.6 33.8 32.7
Wt. % in Liquid Before After 1.87 1.81 1.76 1.81 1.76 1.68 1.59 1.68
Av. 1.84 1.78 1.72 1.63
Wt. yo in Vapor 2.70 2.58 2.45 2.29
70
GO
502.0 471 .O 449.0
471 .O 449.0 429.0
31.0 22.0 20.0
2.05 2.01 1.98
2.01 1.98 1.95
1.74 2.03 2.00 1.97
2.50 2.61 2.57 2.55
733 740 741 741
15 20 15 15
929.2 965.4 369.0 356.4
908.6 948.0 356.4 343.0
20.6 17.4 12.6 13.4
4.46 7.99 10.02 10.01
4.46 7.99 10.01 9.98
2.00 4.46 7.99 10.02 10.00
2.58 5.14 8.52 10.55 10.4
10.0
739 739 739
15 15 15
267.3 258.2 250.3
258.2 250.3 244.2
9.1 7.9 6.1
10.03 10.02 10.01
10.02 10.02 10.00
10.02 10.01 10.00
10.45. 10.35 10.35 10.25
15.05 15.30 15.31 15.35 15.39
10.0 15.05 15.30 15.30 15.33 15.37
19.68 19.71 19.75 19.79
15.3 19.67 19.70 19.73 19.77 19.7 19.73 19.79
18.3 18.7 18.5
744 744 744
741 743 743 743 743 745 745 745 745
80
15 60 60 60 60 10 10 10 10
400.0 503 .O 490.0 480.0 466.3 301.5 295.5 289.6 284.3
393.8 490.0 480.0 466.3 453.5 295.5 289.6 284.3 278.1
6.2 13.0 10.0 13.7 12.8 6.0 5.9 5.3 6.2
15.05 15.29 15.30 15.31 15.35 19.66 19.68 19.71 19.75
__
10.35 14.55 14.3 14.2 14.2 14.2
__
14.2 18.3 18.2 18.3 18.3
--
747 747
55 65
407.0 389.2
389.2 372.7
17.8 16.5
19.72 19.75
19.75 19.82
749 749
60 60
987.0 972.3
972.3 957.0
14.7 15.3
24.53 24.64
24.54 24.55
19.75 24.53 24.55
18.6 22.9 22.9
745 745 745
10
397.0 390.6 386.2
390.6 386.2 380.8
6.4 4.4 5.4
24.45 24.48 24.50
24.48 24.50 24.50
24.55 24.47 24.49 24.50
22.9 23.2 23.2 22.7
28.86 28.88 28.95 29.00 29.04 29.05
24.5 28.86 28.87 28.91 28.97 29.02 29.05
23.1 25.9 25.9 25.9 25.8 25.8 25.9
739 739 739 739 739 739
10 10
10 10 10 10 10 10
396.7 393.1 389.5 384.5 380.2 374.5
the desired reflux ratio for a specified time t o attain equilibrium, returning all the reflux condensate and distillate t o the still; collect sa.mples by manipulation of stopcocks F and L; reverse F and L, so that the distillate and condensate were returned to the: still; and then change the reflux ratio for the next series of samples. The partial condensation which occurred was obtainedbyair cooling, thereflux condensate remaining a t or very near the boiling point.
Figure 9. Partial Condensation Apparatus
669
The data a t atmospheric pressure, given in Tables V and VI and Figures 8 and 10, indicate that (a) when aqueous formaldehyde solutions are distilled with fractionation of the vapors, the distillates contain more formalde.hyde than the liquid-vapor
393.1 389.5 384.5 380.2 374.5 369.7
3.6 3.6 5.0 4.3 6.7 4.8
28.86 28.86 38.88 28.95 29.00 29.04
--
-_
--
--
--
28.95
25.9
equilibrium values, the ratio of condensate to distillate (the reflux ratio) determining the degree of increase in the formaldehyde content of the distillate; (6) the vapors of constant composition previously obtained, which were erroneously called azeotropes, were obtained because of the fact that as the degree of partial condensation is increased, the liquid-vapor curve is raised and crosfies the 45" diagonal a t higher and higher points. From (a) and ( b ) it would follow that no azeotrope exists during distillation a t atmospheric pressure. That the formaldehyde concentration in the total vapor edolved is somewhat richer than the equilibrium value, indicates a slight fractionation in passing through the vapor tube, C . From these data it can be seen that the diverse results obtained by previous investigators can be related to two phenomena: The ratio of condensate to distillate determines the shape of the liquid-vapor curve and hence the formaldghyde content of the distillate; and furthermore, the shape of the liquid-vapor curve is such that as the curve is raised, it crosses the 45' diagonal a t correspondingly higher and higher points. Since each type of apparatus will have a different but constant area for air cooling and therefore fairly constant fractionation characteristics, the reflux ratio will remain reasonably constant for each type of apparatus. During each of the previous distillation investiga-
.
I N D U S T R IiA L A N D E N G I N E E R I N G C H E M I S T R Y
670
Vol. 40, No. 4
OF AQUEOUSFORMALDEHYDE SOLUTIONS AT ATMOSPHERIC PRESSURE WITH FRACTIONAL CONDENSATION TABLE VI, DISTILLATION
Total Barom., Equil. Mm Period, ~
g
744 744 744 744 744 744 737 737 737 737 737 737 739 739 739 739 739 739 739 739 739 739 745 745 745 745
’ Hours
0.25 0.67 0.83 1.51 1.93 2.35 16.0 16.25 16.5 18.0 18.17 18.33 1.0 1.17
Wt. % Formaldehyde
Temperature Liquid, Vapor, OC.
99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5
2.33 2.5 3.17 3.27 3.89 3.97 4.55 4.72 16.0 16.08 16.67 16.75
99.5 99.5 99.5 99.5 99.5 99.5 99.5 99.5 100.0 100.0 100.0 100.0
745 745
17.33 17.42
745 745
20.0 20.08
100.0
746 746 746 746 737 737 737 737 737 737 737 737 737 737
97.5 97.5 97.5 97.5 97.5
719.5 678.0 635.0 918.0 878.6
678.0 635.0 592.8 878.6 838.7
Grams of Vapor Refl. Dist. 16.84 33.31 4.25 31.51 2.09 31.85 11.48 29.00 8.12 29.65 6.4 29.06 18.30 44.35 18.31 44,87 18.85 44.86 33.19 33.87 33.88 28.07 28.39
Vapor Rates, G,/Min. Refl. Dist. 3.33 1.68 3.15 0.42 0.19 2.9 1.15 2.9 2.88 0.79 2.91 0.64 2.95 1.22 1.22 2.99 1.26 2.99
16.0 16.67 16.75 17.25 17.33 18.42 18.75 1.25
1.33
2.42 2.52 4.02 4.10 4.68 4.76 5.35 5.41
100.0 100.0 100.0 100.0
100.0 100.0
97.5 97.5 97.5 97.5 97.5 97.5 97.5 97.. 5 97.0 97.0 97.0 97.0
838.7 803.2 767.2 742.0 713.6 687,9 662.4 629.7 828.0 811.1 973.8 775.3
803.2 767.2 742.0 713.6 687.9 662.4 629.7 597.2
811.1 793.8 775.3 756.7
28.07 28.11 16.85 19.53 13.57 13.68 27.36 27.64 14.76 14.90 14.54 14.69
100.0 100.0 100.0
100.0 100.0 100.0 100.0 100.9
Init. 2.11 2.07 2.03 2.00 1.97 1.94 4.74 4.71 4.68
I n Liquid End 2.07 2.03 2.00 1.97 1.94 1.92 4.71 4.68 4.63
2.09 2.05 2.02 1.98 1.96 1.93 4.72 4.70 4.65
In total In In material refl. dist. distilled 2.75 1 . 9 4 4.31 7.92 2.77 2.07 11.4 2.77 2.20 2.62 1 . 7 6 4.74 5.01 2 .44 1.73 2.40 1.75 5.34 8.94 5.30 3.79 5.13 3.68 8.70 8.85 5.24 3.71
4.69 4.61 4.59 4.55
3.73 3.76 3.70 3.70
8.83 10.0 10.3 10.2
4.58 7.95 7.93
3.71 6.81 6.77
10.2 13.5 13.4
5.03 8.72 8.66
6.79 6.78 6.75
13.45 14.75 14.65
8.69 8.87 8.50
Av.
--
5.22 5.02 5.05 5.01
8.29 8.69 8.70
1.23 0.83 0.87 0.87
2.4 4.0 3.9 3.9
11.30 11.46
3.37 2.81 2.84
0.86 1.13 1.15
3.9 2.5 2.5
1.14 0.84 0.79
2.5 3.3 3.5
7.91 7.85
7.85 7.81
7.94 7.88 7.83 7.85 7.79 7.76
6.76 6.76 6.66
14.7 12.7 12.65
8.68 8.73 8.55
7.78 7.73 7.69
6.71 12.7 6.35 11.15 6.30 11.05
8.64 8.61 8.52
6 . 3 2 11.1 7.0 15.65 6.8815.5
8.57 8.40 8.18
8.43 7.91
2.82 2.81 2.81
_-
~
-_
4.63 4.60 4.57 7.97 7.94
4.60 4.57 4.54 7.94 7.91
-_
_-
--
--
2.81 2.81 2.80
0.82 1.39 1.27
3.4 2.0 2.2
7.81 7.78
7.78 7.74
12.11 11.82
2.81 2.71 2.73
1.33 2.42 2.36
2.1’ 1.1 1.2
7.74 7.71
7.71 7.68
5.32 4.88
2.72 2.74 2.76
2.39 0.53 0.49
--
1.2 5.2 5.6
--
7.68 7.64
7.64 7.62
7:71 7.66 7.63
_-
--
2.15 2.43
2.75 2.95 2.98
0.51 0.43 0.48
5.4 6.9 6.1
15.3 15.3
15.3 15.3
7.65 15.3 15.3
6.94 14.6 14.7
15.6 25.0 24.6
8.29 15.9 16.2
3.94 3.91
2.96 2.91 2.94
0.45 0.79 0.78
6.5 3.7 3.7
15.3 15.2
15.2 15.2
15.3 15.2 15.2
14.6 14.2 14.1
24.8 22.65 22.3
16.05 16.0 15.8
0.79 2.25 2.35
14.1 13.1 12.9
22.5 18.9 18.7
15.9 15.6 15.5
8.33 8.90
-
_-
-
__
--
_-
-- __ -
--
.
__
__
3.7 1.3 1.2
15.2 15.2
15.2 15.2
15.2 15.2 15.2
_-
97.0 97.0
756.7 731.0
731.0 705.0
14.48 14.24
11.26 11.75
2.93 2.90 2.85
97.0 97.0
705.0 689.2
689.2 673.5
14.08 14.12
1.68 1.64
2.88 2.82 2.82
2.30 0.33 0.33
1.2 8.5 8.5
15.2 15.2
15,2 l5,2
15.2 15.2 15.2
13.0 14.4 14.2
18.8 25.5 25.3
15.55 15.6 15.4
0.33
2.14 4.16 4.33
2.82 2.94 2.92 2.91
0.5
8.5 6.8 3.5 3.4
25.4 25.5 25.6
25.5 25.6 25.7
15.2 25.4 25.6 25.7
14.3 20.7 20.2 20.1
25.4 29.7 27.6 27.1
15.5 22.0 21.9 21.7
12.68 12.11
2.91 3.28 3.39
0.84 2.54 2.54
3.4 1.3 1.3
25.9 26.0
25.6 25.8 26.0
20.1 19.5 19.6
27.4 25.05 25.25
21.8 22.0 21.9
2.01 2.54 5.55 5.58
3.33 2.84 2.92 2.86 2.83
2.54 0.40 0.51 1.11 1.11
1.3 7.0 5.76 2.5 2.5
26.1 26.2 32.2 32.4
25.9 26.0 26.1 32.1 32.3
19.5 20.9 20.7 24.7 24.6
25.15 30.4 29.7 29.3 29.3
21.96 22.0 22.0 26.0 25.9
1.79 1.52
2.85 3.1 3.15
1.11 0.3 0.25
2.5 10.5 12.5
32.5 32.7
32.2 32.4 32.6
24.6 25.8 25.9
32.7 33.3
25.95 26.5 26.4
10.71 10.36
3.12 3.04 3.10
0.27 2.14 2.07
11.5 1.4 1.5
33.0 33.2
32.5 32.8 33.1
25.8 25.0 25.1
33.0 28.6 28.6
26.45 26.5 26.5
2.50 2.49
3.06 3.05 3.14
33.5 33.7
33.0 33.4 33.6
25.0 26.0 26.1
28.6 32.1 32.1
26.5 26.8 27.0
16.17 16.55
3.10 2.85 2.83
33.5 33.9 34.3
26.0 32.1 28.4 25.4 25.25 28.0
26.9 27.0 27.1
34.1
25.3
27.05
97.0 97.0 97.0 97.0 97.0
920.0 885.7 867.0 848.1 819.0
902.8 867.0 848.1 819.0 790.0
14.67 14.59 14,53 16.38 16.94
_-
~
100.0 100.0 100.0 100.0
Reflux Ratio, R/D 2.0 7.4 15.2 2.5 3.6 4.5 2.4 2.4 2.4
2.98 3.32 3.39 3.39
~
100.0 100.0
746 746 746
OC.
98.5 98.5 98.5 98.5 98.5 98.5 97.5 97.5 97.5
Grams of Soln. Init. End 893.0 943.0 811.6 847.3 811.6 777.7 777.7 737.2 737.2 599.4 663.9 699.4 846.3 909.0 783.2 846.3 719.5 783.2
96.5 96.5 96.5 96.5 96.5 96.5 97.0 97.0
790.0 773.8 713.0 693.1 673.4 653.0 632,6 606.7
773.8 757.6 693.1 673.4 653.0 632.6 606.7 580.8
14.22 14.62 14.32 14.13 18.56 18.92 15.21 15.52
-
0.83 0.86
-
--
_-
--
25.7 25.9 26.0 26.1 32.0 32.2 32.3 32.5 32.7 33.0
~
96.5 96.5 97.0 97.0
580.8 663.1 554.9 514.5
563.1 544.9 514.5 483.8
15.24 15.69 14.24 14.17
-2.84
tions, the vapor composition probably climbed to the particular enriched vapor level and there occurred what appeared to be the “azeotrppe’’ or the apparent constant boiling mixture obtained by that investigator.
2.10 0.5 0.5
1.5 6.1 6.3
0.5
6.2
3.23 3.31
0.9 0.9
3.27
0.9
33.2 33.5 33.7 34.1
34.1 34.5
_-
__
_-
-- -29.3 -_
_-
__
__ _-
_ I
-28.45
If as usual we define an azeotrope as a system a t a fixed pressure in which, at equilibrium, the vapor and the boiling liquid in intimate contact with each other have the same composition (total formaldehyde), it can be said that the data of Figure 4 show
April 1948
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
67 1
equilibrium involving only methylene glycol and water instead of total formaldehyde and water (in which the methylene glycol content ' of the vapors is greater than the methylene glycol content of the liquid) could adequately explain these enrichment phenomena. The data below indicate rather conclusively however, t h a t vapor enrichment is not due t o a favorable liquid-vapor equilibrium involving methylene glycol and water, 1. Some quantitative d a t a on vacuum distillation, determined by Korzhev and Rossinskaya (14), are shown in Table VII. In order to check these data on distillations of aqueous formaldehyde solutions under reduced pressure, three runs were made a t 20mm. pressure, in the same Wilkinson and Gibson apparatus (Figure 7) used for determining the data on fractional distillation. This apparatus was satisfactory for low 0 4 8 12 16 20 24 28 32 26 40 44 46 temperature work, because p a r t k l oondenWT. O/o FORMAIREHYDE I N LQUl D not Occur at room temperature* Figure 10. Partial Condensation of Aqueous F o r m a l d e h y d e Vapors The results are shown in the fist three lines of Table VII. These data and the data. of that no azeotrope exists for the total formaldehyde-water system Korzhev and Rossinskaya (14) on the distillation of formaldehyde solutions at 20-mm. pressure are in substantial agreement. at atmospheric pressure. It has also been demonstrated by Ledbury and Blair (26)that These data do not show in detail the basic mechanism redistillation of aqueous formaldehyde solutions .under various sponsible for the enrichment of the vapors. However, they do pressures higher than atmospheric yields .distillates which are demonstrate how fractionation of the vapors can be used to richer than the residue, the richness of the distillate increasing obtain distillates of various degrees of richness and they explain with increase in pressure. These data are shown as part of the confusing distillation data obtained by previous investiTable VIII, together with pertinent data selected from the present gators. investigation, Auerbach and Barschall ( 2 ) apparently used distillation equipAlthough the degree of partial condensation is probably not ment in which very little or no fractionation of the vapors was the same in every case, the qualitative effect of preasure may be permitted and the liquid-vapor curve they obtained is similar obtained by examination of the data shown in Table VI11 for the to the liquid-vapor equilibrium curve based on total formaldehyde pressure range of 20 mm. to 100 pounds per square inch gage. so that the distillates were always weaker than the residue. Walker (30)has shown that the methylene glycol content of Wilkinson and Gibson (32) used distillation equipment in aqueous solutions of 0 to 4% concentration does not change which a minimum of fractionation occurred; the liquid-vapor greatly from 20" t o 100" C., while Auerbach and Barschall (g) curve they obtained was slightly higher than the liquid-vapor have shown that the formaldehyde in these solutions at 20' C. equilibrium curve and i t crossed the 45' diagonal a t 8 to 10% is almost 100% in the form of methylene glycol. Therefore, liquid formaldehyde concentration. the great increase in the formaldehyde (or methylene glycol) The other investigators (4, 6, 14, 16, 91) used equipment with concentration of the distillates obtained by the distillation of low which they obtained increasing degrees of fractionation and the concentrations at atmospheric or higher pressure over that liquid-vapor curves were raised correspondingly, so that they crossed the 45" diagonal a t 11 to 12, 23, 30, and 30 to 3370, respectively. TABLE VII. CONCENTRATIONS OF FORMALDEHYDE IN LIQUID Zimmerli (36,36),who used an exceedingly high column with a AND GASPHASES FOR SOLUTIONS BOILING AT 20-MM. PRESSURE very slow rate of distillation, obtained the highest degree of pressure, .HCHO Wt. %.In, HCHO wt. %in fractionation and the curve was raised to such a degree that Mm. Hg Liquid Vapor Source of Data. all vapor coinpositions up to about .%Yo formaldehyde were above 20 2.1 0.16 This investigation the 45 " diagonal. Walker (29,S I ) , by using a very high degree of 20 4.4 0.33 This investigation 20 6.0 0.42 This investieation partial condensation, by chilling the reflux condensate to a low 20 6.82 0.46 Korrhev a n a Rossinskaya 14 temperature, and by removing the reflux condensate as quickly 20 16.9 1.19 Korzhev and Rossinskaya 1141 20 28.2 4.66 Korzhev and Rossinskaya (14) aa possible from contact with the remaining vapors, so that the reflux condensate had little opportunity to approach the liquidTABLEVIII. INFLUENCE OF PRESSUREON DISTILLATION OF vapor equilibrium value, was able to obtain results similar to FORMALDEHYDE SOLUTIONS those of Zimmerli. ELIMINATION OF A FAVORABLE METHYLENE GLYCOLWATER LIQUID-VAPOR EQUILIBRIUM AS A CAUSE O F VAPOR ENRICHMENT
The data presented here on the distillation of aqueous formaldehyde solutions a t atmospheric pressure show how fractionation can be effective in causing vapor enrichment. Since Walker (30) has shown that the partial pressure of formaldehyde in aqueous solutions is due only to the methylene glycol content of these solutions, it is apparent that a favorable liquid-vapor
Pressure 20 mm. 500 mm. * 760 mm. 760 mm. 20 lb./sq. in. 40 lb./sq. in. 60 lb./sq. in, 80 lb./sq. in. 100 Ib./sq. in. 20 mm. 20 mm. 760 mm.
Approx. Temp., "C. 22 90 100 100 126 152 153 162 170 22 22 100
Wt. %, Wt. %, HCHO In HCHO In Liquid 2.1 2.1 2.1 2.1 2.1 2.1 2.1 2.1 2.1 6.0 6.82 6.82
Vapor 0.16 I1.47 2.1 3.2 4.6 5.4 6.2 7.4 7.6 0.42 0.46 6.6
Source of Data
This investigation! Table VI1 Korrhev and Rossinskaya(l4) This investigation, Figure 4
672
INDUSTRIAL AND ENGINEERING CHEMISTRY
?ABLE
Total Equil. period, Hours 0.25 0.92 1.33
Ix.
DISTILLATION OR AQUEOUS
Intermediate Equil. Dist. Temperature Period, Time, Liquid, Vapor, Hours Min. OC. ‘C. 10 90 87.5 0.25 10 90 87.5 0.5 10 90 87.5 0.28
F O R a f A L D E H Y D E S O L U T I O N S AT
Grams of Soh. Init. End 908.0 875.2 875.2 843.9 843.9 801.5
Grams of Vapor Refl. Dist. 22.49 10.36 22.96 8.34 22.55 19.88
obtained by distillation a t about 20” C. a t 20-mm. pressure is probably not to be attributed to variation in the methylene glycol concentration of the solutions. 2. Examination of both the liquid-vapor equilibrium curve (Figure 4) and the fractional distillation curves (Figures 8 and 10) indicates that the vapors evolved from solutions containing 0 to 4% formaldehyde are probably weaker in formaldehyde than the liquid, and that after fractionation the vapors are richer than the liquid. However, the differences between the liquid and vapor concentrations at equilibrium are so slight that the significance of the differences involved may well be questioned. It is desirable to show by a more significant difference that the initial vapors evolved from a formaldehyde solution of 0 to 4% concentration are weaker in formaldehyde than the liquid, whereas after enrichment has occuired, the vapors are appreciably richer than the liquid. This information can best be shown by data obtained a t a pressure somewhat lower than atmospheric, as a greater $ifferenee exists between vapor and liquid composition as the pressure is lowered. For these data a run was made a t 500-mm. pressure in the apparatus shown in Figure 9. The method and procedure used were identical with those used for the atmospheric fractional distillation data. These data, given in Table IX, show that a t a reflux ratio of 1.1 the formaldehyde content of the enriched distillate is less than that of the liquid, while at reflux ratios of 2.2 and 2.8 the formaldehyde content of the enriched vapor is greater than that of the liquid. However, in every case the concentration of formaldehyde in the total evolved vapor is appreciably less than the concentration of formaldehyde in the liquid. These data indicate that vapor enrichment is not the result of a favorable liquid-vapor equilibrium involving methylene glycol but must be due to other factors. The experiments carried out here demonstrate that, under their dynamic conditions, distillates stronger than the residue are easily obtained. It therefore appears that in these experiments we are dealing with nonequilibrium conditions; hence dynamic vapor-liquid studies are to be recommended in further investigations of this interesting . system. SUiMMARY AND CONCLUSIO3S
When formaldehyde solutions are distilled at atmospheric pressure under equilibrium conditions where reflux condensation is avoided, the total formaldehyde content of the distillate is always less than the total formaldehyde content of the residue. The liquid-vapor equilibrium curve obtained, based on total formaldehyde, deviates more and more from the 45 diagonal as the content of higher hydrates increases. When formaldehyde solutions are distilled a t atmospheric pressure with fractionation of the vapors, the distillates contain more formaldehyde than the liquid-vapor cquilibrium values, the ratio of condensate to distillate determining the degree of increase in the formaldehyde content of the distillate. ‘ The diverse distillation rcsults obtained at atmospheric pressure by previous investigators were duc t o two phenomena: The ratio of condensate to distillate determines the formaldehyde content of the distillate and the shape of the liquid-vapor curve is such that as the curve is raised, through partial condensation, it crosses the 45’ diagonal a t correspondingly higher and higher points so that various azeotropes seemed to exist.
500-1\1~.P R E S S U R E
Vapor Rates,
G./i\.Iin. Refl. Dist. 2.23 2.30 2.26
1.03 0.833 1.99
WITH
Vol. 40, No. 4 FRACTIONAL CONDENSATION Wt. % Formaldehyde
Ratio
In f Liquid l ~ Init. End -Kv.
~I n refl.
In dist.
In total material distilled
2.2 2.8
2.11 2.13 2.13 2.13 2.15 2.14 2.15 2.19 2.17
1.08
2.30 2.57 1.88
1.47 1.47 1.42
~
~
1.1
1.09 1.00
The evidence presented here shows that there is no azeotrope existing during distillation at atmospheric pressure. Vapor enrichment is due to other factors than a favorable liquid-vapor equilibrium involving methylene glycol and water. The boiling points of aqueous formaldehyde solutions at 760mm. pressure decrease LTith increasing concentration from 100.0 C. for a 0% concentration to a minimum of 98.95’ C. for the concentration range of 20 t o 3570 formaldehyde and t h m increase to 100.0” C. for a 53% solution. An investigation of the phenomena causing vapor enrichment and other properties of formaldehyde vrill be presrnted in a succeeding paper. LITERATURE CITED (1) Aueihach, F.,and Barschall, €I., A r b . Ilaiscrl Gesundh., 27, 7 (1907). Auerbach, F., a n d Barschall, H., A?b. Reichsgesundh., 22, 584 (1905). Baker, E., H u b h a r d , R., H a g n e t , J., and Michalowski, J., IND. ENG.CHEM.,31, 1260 (1939). Blair, E., and Taylor, R., J . Soc. Chem. I n d . , 45, 65T (1926). Bond, H. A., U. S. P a t e n t 1,905,033 (April 2 5 , 1933). Borgstrom, P., J . Am. Chem. Soc., 45, 2150 (1923). Buchj, J., P h a r m . Acta Helv., 6 , 1. (1931). Chem. M e t . Eng., 53, 103 (1946). Davis, H. L., J . C h e m . Education, 10, 47 (1933). D o b y , U., 2. angeu. Chem., 20, 363 (1907). Griswold, J., Andres, D., a n d Klien, V.. Trans. Am. Inst. Chem. Engrs., 39, 223 (1943). Hasche, R . L., U. S. P a t e n t 2,015,180 (Sept,. 24, 1935). Hibhen, 3., J . Am. Chem. Soc., 53, 2418 (1931). Korshev, P., and liossinskaya, I., J . Chem. I n d . (U.S.S.R.), 12, 601 (1935). Lacy, B . S., see A.C.S. Monograph 98, “Formaldehyde,” by Walker, p . 54, New York, Reinhold Publishing Corp., 1944. Ledbury, W., and Blair, E., D e p t . Sei. I n d . Research, Spec. R e p t . 1, 40-51 (1927). a n d Blair, E., J . Chem. Soc., 127, 26, 2832 (1925). Ledhury, W., Lenime, G., Chem.-Ztg., 27, 896 (1903). Othmer, D . I?., IKD. ENG.CHEY.,20, 743 (1928). Ibid., 35, 614 (1943). Reynolds, B. M., U. S. P a t e n t 2,256,497 (Sept. 2 3 , 1941). Schou, S. A,, J . Chem.. P h y s . , 26, 72 (1929). Seiler, IC.,Schweiz. Apoth. Ztg., 73, 710 (1935). Swietoslawski, W., “Ebullionietric Measurements,” pp. 2 2 , 37, 59, New York. Reinhold Publishing Corp., 1945. Taufel, K . , a n d Wagner, C., 2. anal. Chem., 6 8 , 25 (1926). U. S. Tariff Commission, “Synthetic Organic Chemicals, U. S. Production a n d Sales,” Report 148, Second Series (1940). , W-adano, M., Ber., 67,191 (1934). Walker, J. F., “Formaldehyde,” A.C.S. Monograph 98, pp. 48-59, 7 3 , New Y o r k , Reinhold Publishing Corp., 1944. Walker. J. F.. ISD.ENG.CHEY..32. 1016 (1940). Walker: J . F.: J . P h y s . Chem., 35, 1104 (1931). ’ Walker, J. F., 0. S. P a t e n t 1,871,019 (Aug. 9, 1932). Wilkinson, J . , a n d Gibson, I., J . Am. Chem. Soc., 43, 695 (1921). Young, S., ”Distillation Principles and Processes,” pp. 62-76, London, Maemillan Co., 1922. Zawiduki, J., 2 . physik. Chem., 35, 129 (1900). ENG.CHYM.,19, 524 (1927). Zimmerli, A., IND. (36) Zimmerli, A., U. S. P a t e n t 1,662,179 (March 13, 1928). RECEIVED July 12. 1947. This investigation was supported in part by funds granted by the Graduate School of the University of Minnesota.
End of Symposium Chemical Engineering in the Plastics Industry
